I have a couple thoughts to offer for brainstorming . . .
I have a relationship with health department where I think I would call the lead inspector. I would do a comprehensive investigation locally and document the purchase/received/served dates; copy of temperature logs for the cooler where stored; time of order and who prepared and served the food; statement of anyone else, or that no one else reported illness; as well as a brief interview with the people working who may have been in the chain of service to that customer. It would show responsibility, intent to find an answer, and give lots of information in the event someone really makes a big deal about it.
AS to the customer, I would offer sympathy that they got sick, assure the investigation would be done to see assure proper procedures are being followed, and offer a replacement or enticement to return. If you have a relationship with this customer, you could instead even call after your investigation to tell them you’ve contacted health department and gone over the whole process to be sure your place is safe . . . do not admit or insinuate responsibility for the illness. That is for courts and health department to determine.
On the other side is potential “quick” buggies . . .
Poor hygene and/or handwashing can possibly lead to a mild to moderate case of
Staphylococcus aureus: Causes moderate to severe illness with rapid onset of nausea, severe vomiting, dizziness, and abdominal cramping. These bacteria produce a toxin in foods such as cream-filled cakes and pies, salads (most at risk are potato, macaroni, egg, and tuna salads, for example) and dairy products. Contaminated salads at picnics are common if the food is not chilled properly.
Grimy hair wasn’t washed today, hand get touched to the hair a few times during the shift, touch the food and container when being packaged, it sits in the container for 20-30 minutes waiting to be be picked up, then 20 minutes before being consumed. It can happen.
No real way to prove it came from your place, either. It could have been from the grocery bags at the place they stopped for butter and eggs.